1. Field of the Invention
The invention relates to articles formed from multiple metal layers used in electronic devices, and more particularly, to multiple metal layers attached to substrates to provide circuitry or circuit boards. The invention is especially advantageous when applied to manufacturing processes for thin circuit boards, particularly circuit boards with physically durable structures, including air bridges.
2. Background of the Art
A technique for forming a tri-layer metal structure is described in U.S. Pat. No. 5,428,250 to Ikeda et al. The tri-layer metal structure is formed on a glass substrate. The first layer is a Taxe2x80x94Mxe2x80x94N film, the second layer is a Ta film and the third (top) layer is a Taxe2x80x94Mxe2x80x94N film, where M is at least one atom selected from the group consisting of Molybdenum, Niobium, and Tungsten.
U.S. Pat. No. 5,153,754 to Whetten described a tri-layer metal structure formed on an LCD substrate where the first layer is a titanium (Ti) film, the second layer is a molybdenum (Mo) or aluminum (Al) film, and the third (top) layer is a titanium (Ti) film. In addition, column 6, lines 56-70 describe a process to taper etch the tri-layer metal structure. When the second layer is a molybdenum film, the tri-layer structure is formed by wet etching the titanium first layer with fluoroboric acid (HBF4), wet etching the molybdenum second layer with PAWN (phosphoric acid, acetic acid, water and nitric acid), and dry etching the titanium third layer in a plasma barrel etcher with an atmosphere of CF4 and O2 (or SF6 and O2). When the second layer is an aluminum film, the tri-layer structure is formed in a single etch step by an RIE etch of BCl3, CCl4 and O2.
U.S. Pat. No. 5,464,500 to Tsujimura et al. describes a tri-layer metal structure formed on a glass substrate. A silicon oxide layer is formed on the glass substrate. The first metal layer of Aluminum (Al) is formed on the silicon oxide layer. The second metal layer of aluminum oxide is formed on the first metal layer. The third metal layer of molybdenum is formed in the aluminum oxide layer. Beginning at column 3, line 60, a process for taper etching the tri-metal layer is described. As a result, the cross section of the first metal layer of aluminum is formed with a taper angle.
U.S. Pat. No. 4,824,803 to Us et al. describes a tri-layer metal structure formed on a glass layer wherein the first metal layer is a titanium (Ti) film, the second metal layer is an Aluminum (Al) film, and the third metal layer is a titanium film. As described beginning at column 2, line 45, the tri-metal structure is formed in a single RIE etch step of a chlorine based chemistry. As shown in FIGS. 1a and 1b, the RIE etch step results in a non-tapered structure with vertical sidewalls.
U.S. Pat. No. 4,650,543 teaches a GaAs FET electrode wiring layer or bonding pad having a three-layered structure of Au/Pt/Ti or a two-layered structure of Al/Ti. The electrode wiring layer or the bonding pad is sometimes formed by a wet etching method but mainly by a lift-off method. A method of forming a bonding pad by wet etching is described. In this case, an insulating film is formed on a GaAs semiconductor substrate by CVD, and thereafter a contact hole is selectively formed in the insulating film. A metal film for forming a bonding pad is deposited on the overall surface of the substrate, and a resist pattern is formed thereon. Finally, the metal film is etched by wet etching using the resist pattern as a mask so as to form a bonding pad of the metal film on the hole of the insulating film. In this method, since the GaAs semiconductor layer is highly sensitive to chemical treatment, when the wet etching method is used, side etching occurs. For this reason, this method is inappropriate for forming a micropattern such as a gate electrode. Note that in a GaAs FET, a submicron micropattern must be formed. Therefore, a lift-off method was developed for micropatterning. This method is described in U.S. Pat. No. 3,994,758. However, the metal film formed by this method was formed by CVD (Chemical Vapor Deposition) at a low temperature because of a resist film. For this reason, bonding between a metal multilayer and a semiconductor substrate constituting an electrode pattern was inadequate. Therefore, the electrode pattern was easily removed during lifting off or wire bonding, thus degrading the yield in manufacturing of the GaAs FET. This Patent asserted an advance in the technology by the electrode pattern having a multilayer structure selected from the group consisting of Au/WN, Au/W/TiW and Au/Mo/TiW (elements on the left side are positioned uppermost with respect to the semiconductor substrate). In an ion milling technique used in that invention, etching is performed by bombarding a member to be etched with ions of an inert gas such as Ar or Ar+O2 gas using a shower or beam type device. This technique is inert dry etching and is also called ion etching. This ion milling technique has been disclosed in, e.g., Solid State Tech. March 1983, Japanese Edition p. 51 to 62. In a reactive ion etching technique, by using a parallel-plate, microwave or ion-shower type device, dry etching is performed by reactive plasma using a reactive gas mixture such as CF4 +O2 or CF4 +Cl while activating a member to be etched using an inert gas such as Ar gas.
U.S. Pat. No. 5,912,506 addresses perceived problems of
(a) thinning of additional metal layers crossing the edges of the multi-layer metal structure;
(b) shorts or pinholes formed in one or more insulator layers above multi-layer metal structure due to near vertical or undercut edges; and
(c) controlling the effective width of the multi-layer structure when using an extended non-directional overetch. These problems are variously addressed by the invention of that Patent. A multi-layer metal sandwich structure formed on a substrate includes a first metal layer formed on the substrate and a second metal layer formed on the first metal layer. The second metal layer has tapered side walls. The width of the first metal layer is different than the width of the second metal layer at the interface of the first metal layer and the second metal layer. The multi-layer metal sandwich may also include a third metal layer formed on the second metal layer. The second metal layer may also be substantially thicker than the first or third metal layers. A method for forming the multi-layer metal sandwich with taper and reduced etch bias on a substrate includes the steps of forming a three-layer sandwich of metal on the substrate by forming a first metal layer on the substrate, forming a second metal layer on the first metal layer, and forming a third metal layer on the second metal layer. A resist pattern is formed on the three-layer sandwich, wherein the resist pattern defines etch areas in the three-layer sandwich. The etch areas are exposed to a first etchant that taper etches the second metal layer while not attacking the first metal layer. The etch areas are then etched using a directional etch process, which etches the first metal layer. The resist pattern in then removed. The third metal layer may be removed. Preferably, the first metal layer is titanium (or a titanium alloy), the second metal layer is Aluminum (or an Aluminum alloy), and the third metal layer is Molybdenum (or Molybdenum alloy) or Copper (or a Copper alloy) or other refractory metal (or alloy). In this case, the first etching process for taper etching the aluminum second layer utilizes a wet etchant that is a mixture of phosphoric acid, nitric acid, acetic acid, and water, and the second etching process for etching the titanium first metal layer utilizes an RIE etching process.
U.S. Pat. No. 3,801,388 teaches a method of manufacturing a printed circuit board with crossover circuits and a process for its manufacture. A tri-metal sub-element comprising copper-iron-copper (with nickel as an alternative metal layer) is etched, and an etched copper layer is adhered to a substrate. Ammonium persulfate and ferric chloride solutions are described as etch solutions for copper layers, and a solution of oxalic acid and hydrogen peroxide is disclosed for the iron layer.
Each of these references emphasizes the fact that each multiple layer element has its own unique properties and tends to require unique processing solutions and processing controls. It is desirable to be able to provide different multi-layer sandwiches with unique properties, both within individual layers and within the functional ability of the composite. However, each such different system requires fundamental investigation of the properties and the processing necessities.
A multi-layer metallic element useful in the formation of circuitry comprises copper/aluminum/copper sub-elements (hereinafter referred to as xe2x80x9ctri-metal sub-elements,xe2x80x9d even though more than three layers may be present) which are etched to form individual sub-elements of circuits. These individual sub-elements are then electrically connected (e.g., with posts, vias, electrically conductive-material filled holes [e.g., solder fills a drilled or etched through hole] or plated through-holes) to form larger circuit elements. Circuitry may be formed by any method including the following, not necessarily sequential, steps of a) providing at least one (preferably two or more) of the tri-metal sub-elements, b) providing a separator sub-element between the tri-metal sub-elements, c) drilling, plating, etching, abating, inserting posts, or coating through-holes to electrically connect at least two of the tri-metal sub-elements (typically) or d) one tri-metal sub-element is adhered to a support before further etching is performed (e.g., in a separator sub-element or ground plane; that is a non-conductive support or a conductive support), e) providing a resist layer on at least one surface of at least one of the tri-metal sub-elements, f) exposing or otherwise activating the resist (e.g., exposing a radiation sensitive resist to appropriate radiation in an image-wise pattern, thermally exposing a thermal resist in an image-wise pattern, printing on a resist in an image-wise pattern, etc.), f) developing the resist pattern to expose an underlying surface of at least one tri-metal sub-element (of course one resist layer can expose only one underlying trimetal sub-element at a time), g) etching at least one layer of the tri-metal sub-element through openings in the developed resist layer, and h) stripping the resist from the surface. Typically, the etched sub-element is adhered to a support before the etching of the central layer (e.g., the aluminum) is done (e.g., on a separation element or ground plane). After the first layer has been etched, the next layer (the aluminum) may be etched, with the copper layer remaining as at least part of a resist surface. The remaining aluminum after the second resist etching and/or the copper may be coated with an organic solderability preservative or metallized (e.g., silvered) to increase its conductivity and/or its ability to be soldered. By this method, not only may conventional circuits be manufactured, but air bridges may also be constructed within the flow of process steps. The second exterior metal layer of the tri-metal subelement (e.g., the second copper layer) may be etched before or at the same time as the first metal layer (first copper layer), after the etching of the first layer but before etching of the interior aluminum layer, or after the etching of the first metal layer but after etching of the aluminum layer. Solder mask may be added at various stages during the processing for the purpose of providing insulation, underfilling for air bridges, and encapsulation of edges, traces, and the like. Flexible, rigid, and segmented flexible and rigid (rigid-flex) circuit boards my be manufactured according to the invention by the selection of appropriate layer thicknesses and support layers. Multiple numbers of etched tri-metal subelements may be directly laminated together without intermediate supports or ground plane layers. This would be accomplished by applying adhesive to an outer surface (or remaining outer surface) of the etched tri-metal subelements. The adhesive would be a dielectric for best performance of the bonded multiple tri-metal subelements without an intervening distinct layer.